1. Field of the Invention
The present invention is in the field of forming protective coatings on metals, and more particularly the formation of such coatings through phosphatization.
2. The Prior Art
Phosphatization of steel has become increasingly important among processes for protection from corrosion, especially when followed by chromium plating or to assure good anti-corrosion protection and adherence of paints applied afterwards.
Many processes have been disclosed which result in the formation of metallic phosphates, usually zinc, iron and manganese having various crystalline or amorphic structures. Certain modifications to the processes seek to vary the speed of formation of phosphatic layers (including when cold) by means of accelerators and other auxiliary products such as humectants, dispersions and passivators among others. However, all these processes require a three part treatment, that is: first a phosphating bath; second a washing to eliminate non-combined acid products; and third a chromic acid based bath to passivate the phosphate layers and increase corrosion resistance.
It is therefore obvious that phosphating treatments heretofore known require costly, complex installations and that the treatment time is prolonged.
Where zinc or aluminum surfaces are to be treated, other systems are used based typically on phosphates and chrome coatings and accelerators for their attack, which procedures must be followed by washing to eliminate any uncombined products.
In all cases, prior procedures, by requiring that the surface of the metal be attacked to form protective substances insoluble in water, required as a corrolary that the procedure be carried out at a relatively high temperature and for a relatively protracted period of time since the compounds formed are usually soluble in water, becoming insoluble only upon consumption of acid on the surface and the consequent progressive formation of metallic ions which are being added to the interfacial film.
Broadly stated, this application relates to a method for forming in a single application a corrosion resistant, reticulated coating on metals, such as iron, zinc, aluminum, cadmium, steel, cooper and alloys thereof, and which provides, in addition, an adherent surface for paint. The method includes the steps of subjecting the metal to be treated to a treatment solution obtained by dissolving in phosphoric acid or a derivative thereof, metal salts selected from the group of zinc, manganese, iron and lead to provide a solution including primary phosphates of said metals, adding thereto an organic reducing agent, partially oxidizing the resultant composition through the addition of chromic acid or its salts, to produce in solution trivalent chromium ions, and applying said solution, preferably but not necessarily in heated condition, to the said metal to be treated.
The treated metal surface is thereafter dried to provide thereon a reticulated film strongly adherent to the base metal.
Preferably a polymeric compound in solution, emulsion or dispersion is added to the treatment composition, whereby polymeric elements are incorporated in the film, improving the anti-corrosive properties and rendering the same especially receptive to subsequent paint coatings.
The drying step is preferably carried out in a heated environment, augmenting the quality of the coating or film.
The treatment solution is acid, preferably in the range from about pH 1 to 3.
From the foregoing it will be appreciated that the instant method offers the innovation, as compared to phosphatizing procedures heretofore known, or eliminating the rinsing and chromic passivation steps. Thus, the procedure is greatly simplified in that the treatment consists of only one impregnation, dipping or coating, combining phosphating, washing and passivation in a single step. By thus eliminating treatment stages, savings in installation costs, time, and increased production output are assured.
Additionally, the operation may be carried out when cold, thereby offering the possibility of energy conservation.
Whereas conventional phosphating methods offer limited protection against corrosion, due to rapid reoxidation, the protection against corrosion given by the instant process is far superior, due both to the film forming and water repellent characteristics of the coating.
Further applications of rust preventitive oils and paints need not be immediately effected due to the durable nature of the coating. The surface provides improved adherence to subsequently applied paints.
An element of novelty of the procedure which both simplifies application and increases protection lies in the characteristic of the admixed components to form a reticulated film, which film engulfs all of its components and any residual acidity, with the resultant passivation closing the pores in the film.
Without limitation to any specific theory, the improved operation of the procedure, and its ability to eliminate rinsing and chromic passivating stages, is considered to be a consequence of the formation in situ of trivalent chromium salts which have a complex, closed structure, since during the reaction trivalent chromium is being generated. The film obtained is not hygroscopic, in contrast to the films generated when traditional zinc or iron phosphates are employed in the usual manner. The film is insoluble in water, especially when dried at high temperatures and offers notable anti-corrosive protection.
In contrast to traditional procedures, the phosphates generated in accordance with the process do not have to become insoluble by progressive attack of the metal, the film being deposited and becoming insoluble during drying. The thickness of the phosphate coat will depend on the intensity of the attack on the metal, according to the temperature and time of treatment.
In any case, given the high acidity of the treatment solution, the metallic surface is immediately attacked, with consequent conversion into phosphate. The treatment therefore does not require protracted immersion time for attack to take place, nor the application of high temperatures. The thickness of the film will depend only on the concentration of the solution.
As previously noted, multiple advantages flow from the present procedure, including high speed of application, ability to permit cold application, the permitting of simpler and less expensive installations, and the formation of an improved protective film.
The conventional treatments for aluminum currently include hexavalent chromium compositions. The remnants of these compositions are very high pollutants of water, thus causing very expensive installations for purification. The composition we are dealing with does not always require presence of hexavalent chromium. The chromium can all be in the form of trivalent chromium, which will be eliminated from the remnant product by precipitation with alkalis, such as lime (O Ca) and subsequent filtration. Another advantage of this process is consequently the reduction of water pollution.
Since compounds of the product remain wholly on the metal, organic polymers may be incorporated in the treatment material, which polymers may be selected to have a structural affinity with the resins which will form part of the paints to be subsequently applied. It is therefore possible to assure that the paints have excellent adherence and the metal may undergo severe deformation without release of the paint.
It is also necessary that the organic polymers be insoluble in water after drying, which can generally be achieved by a heat cure. The most suitable polymers are the acrylics with thermosetting properties. Through the use of polymers the corrosive-resistant properties are improved. Compatible pigments may be added to the polymer mix.
It will thus be appreciated that the resultant film, where polymers are included, acts not merely as a phosphating coating but also serves the function of a primer or first coat of paint.
Particularly when the composition is used in very high concentrations, rendering film thicknesses above 5 microns, and especially from 10 to 15 microns, very important anticorrosive properties are obtained, e.g. up to 300 to 400 hours in a spray salt test at 5%. These values are much higher than those of a primer in a water medium (hydrosoluble). Accordingly, in some cases the coating may perform the functions both of the phosphating agent as well as the first coat of paint, with a consequent reduction in the operations required, as noted below:
______________________________________ Conventional operation described composition ______________________________________ degreasing degreasing rinsing rinsing phosphatization Rinsing passivating drying priming phosphating composition (film 10 microns) curing curing top coat top coat curing curing ______________________________________
Paint adherence is not only increased on traditionally treated materials, such as steel, aluminum and zinc, but also offers good results on tin and its alloys, copper and its alloys and, generally speaking, on the majority of metallic surfaces.
The invention is further directed to a treatment composition adapted to be applied to metallic surfaces for the formation thereon of a film of the type described.
The process consists generally of the following stages: